Method for monitoring a metal layer during chemical mechanical polishing using a phase difference signal

ABSTRACT

A sensor for monitoring a conductive film in a substrate during chemical mechanical polishing generates an alternating magnetic field that impinges a substrate and induces eddy currents. The sensor can have a core, a first coil wound around a first portion of the core and a second coil wound around a second portion of the core. The sensor can be positioned on a side of the polishing surface opposite the substrate. The sensor can detect a phase difference between a drive signal and a measured signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of and claimspriority under 35 USC 120 to U.S. application Ser. No. 09/574,008, filedon May 19, 2000, the entire disclosure of which is incorporated hereinby reference.

BACKGROUND

[0002] The present invention relates generally to chemical mechanicalpolishing of substrates, and more particularly to methods and apparatusfor monitoring a metal layer during chemical mechanical polishing.

[0003] An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive or insulative layerson a silicon wafer. One fabrication step involves depositing a fillerlayer over a non-planar surface, and planarizing the filler layer untilthe non-planar surface is exposed. For example, a conductive fillerlayer can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. The filler layer is thenpolished until the raised pattern of the insulative layer is exposed.After planarization, the portions of the conductive layer remainingbetween the raised pattern of the insulative layer form vias, plugs andlines that provide conductive paths between thin film circuits on thesubstrate. In addition, planarization is needed to planarize thesubstrate surface for photolithography.

[0004] Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is placed against a rotating polishing disk pad or beltpad. The polishing pad can be either a “standard” pad or afixed-abrasive pad. A standard pad has a durable roughened surface,whereas a fixed-abrasive pad has abrasive particles held in acontainment media. The carrier head provides a controllable load on thesubstrate to push it against the polishing pad. A polishing slurry,including at least one chemically-reactive agent, and abrasive particlesif a standard pad is used, is supplied to the surface of the polishingpad.

[0005] One problem in CMP is determining whether the polishing processis complete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness, or when a desired amount of material hasbeen removed. Overpolishing (removing too much) of a conductive layer orfilm leads to increased circuit resistance. On the other hand,underpolishing (removing too little) of a conductive layer leads toelectrical shorting. Variations in the initial thickness of thesubstrate layer, the slurry composition, the polishing pad condition,the relative speed between the polishing pad and the substrate, and theload on the substrate can cause variations in the material removal rate.These variations cause variations in the time needed to reach thepolishing endpoint. Therefore, the polishing endpoint cannot bedetermined merely as a function of polishing time.

[0006] One way to determine the polishing endpoint is to remove thesubstrate from the polishing surface and examine it. For example, thesubstrate can be transferred to a metrology station where the thicknessof a substrate layer is measured, e.g., with a profilometer or aresistivity measurement. If the desired specifications are not met, thesubstrate is reloaded into the CMP apparatus for further processing.This is a time-consuming procedure that reduces the throughput of theCMP apparatus. Alternatively, the examination might reveal that anexcessive amount of material has been removed, rendering the substrateunusable.

[0007] More recently, in-situ monitoring of the substrate has beenperformed, e.g., with optical or capacitance sensors, in order to detectthe polishing endpoint. Other proposed endpoint detection techniqueshave involved measurements of friction, motor current, slurry chemistry,acoustics and conductivity. One detection technique that has beenconsidered is to induce an eddy current in the metal layer and measurethe change in the eddy current as the metal layer is removed.Unfortunately, the proposed eddy current sensing techniques typicallyrequire complex electronics. In addition, the sensors are positioned onthe backside of the substrate. Since the magnetic field of the sensorextends toward the platen, special shielding is needed to prevent themetal platen from interfering with the eddy current measurements.

SUMMARY

[0008] In one aspect, the invention is directed to a sensor formonitoring a conductive film in a substrate. The sensor has a corepositionable in proximity to the substrate, a first coil wound around afirst portion of the core, an oscillator electrically coupled to thefirst coil to induce an alternating current in the first coil andgenerate an alternating magnetic field in proximity to the substrate,and a second coil wound around a second portion of the core. A capacitoris electrically coupled to the second coil, and an amplifier iselectrically coupled to the second coil and the capacitor to generate anoutput signal.

[0009] Implementations of the invention may include one or more of thefollowing features. The oscillator may induce an alternating currentwith a frequency selected to provide a resonant frequency when thesubstrate is not in proximity to the core. The core may consistsessentially of ferrite, and may includes two prongs and a connectingportion between the two prongs. The first coil may be wound around theconnecting portion, and the second coil may be wound around at least oneof the two prongs. The second coil and the capacitor may be connected inparallel. The sensor may be positioned on a side of a polishing padopposite the substrate. The polishing pad may includes an upper layerand a lower layer, and an aperture may be formed in at least a portionof the lower layer adjacent the core. A computer may receive the outputsignal.

[0010] In another aspect, the invention is directed to a chemicalmechanical polishing apparatus. The apparatus has a polishing pad, acarrier to hold a substrate against a first side of the polishingsurface, an eddy current sensor, and a motor coupled to at least one ofthe polishing pad and carrier head for generating relative motiontherebetween. The sensor includes at least one inductor positioned on asecond side of the polishing pad opposite the substrate, an oscillatorelectrically coupled to the at least one inductor to induce analternating current in the coil and generate an alternating magneticfield, and a capacitor electrically coupled to the at least oneinductor.

[0011] Implementations of the invention may include one or more of thefollowing features. A platen may support the polishing pad, and the atleast one inductor may be positioned in a recess in a top surface of theplaten. The platen may rotates, and a position sensor may determine anangular position of the platen and a controller to sample data from theeddy current sensor when the at least one inductor is positionedadjacent the substrate. A recess may be formed in the second side of thepolishing pad. The polishing pad may include a cover layer on the firstside of the polishing pad and a backing layer on the second side of thepolishing pad, and the recess may be formed by removing a portion of thebacking layer. The eddy current sensor may include a core having twopoles positioned adjacent the recess in the polishing pad, and the atleast one inductor is wound around a first portion of the core. The eddycurrent sensor may include a core, and the at least one inductor mayinclude a first inductor wound around a first portion of the core and asecond inductor wound around a second portion of the core. Theoscillator may be electrically coupled to the first coil to induce analternating current in the first coil. The capacitor may be electricallycoupled to the second coil. The oscillator may induce an alternatingcurrent with a frequency selected to provide a resonant frequency whenthe substrate is not in proximity to the core. An endpoint detectionsystem may receive an output signal from the eddy current sensor. Theendpoint detection system may be configured to signal a polishingendpoint if the output signal exceeds a predetermined threshold.

[0012] In another aspect, the invention may be directed to a method ofmonitoring a thickness of a conductive layer in a substrate during apolishing operation. In the method, a substrate is positioned on a firstside of a polishing surface, and an alternating magnetic field isgenerated from an inductor positioned on a second side of the polishingsurface opposite the substrate. The magnetic field extends through thepolishing surface to induce eddy currents in the conductive layer. Achange in the alternating magnetic field caused by a change in thethickness of the conductive layer is detected.

[0013] Implementations of the invention may include one or more of thefollowing features A first coil may be driven with an oscillator at afirst frequency. The first frequency may be a resonant frequency whenthe substrate is not in proximity to the magnetic field. The alternatingmagnetic field may be sensed with a second coil. The second coil may beconnected in parallel with a capacitor. The first coil may be woundaround a first portion of a core, and the second coil may be woundaround a second portion of the core. When the inductor is adjacent thesubstrate may be determined. The inductor may be driven with a firstsignal, and a second signal may be generated from the alternatingmagnetic field. A change in amplitude in the second signal may bedetermined. A change in a phase difference between the first signal andthe second signal may be determined.

[0014] In another aspect, the invention is directed to a method ofchemical mechanical polishing. In the method, a substrate having aconductive layer is positioned on a first side of a polishing surface.An alternating magnetic field is generated from an inductor positionedon a second side of the polishing surface opposite the substrate. Themagnetic field extends through the polishing surface to induce eddycurrents in the conductive layer. Relative motion is created between thesubstrate and the polishing surface to polish the conductive layer. Theeddy currents in the substrate are sensed, and polishing is halted whenthe sensed eddy currents exhibit an endpoint criteria.

[0015] Implementations of the invention may include one or more of thefollowing features. The endpoint criteria may be the eddy currentspassing a threshold strength or leveling off.

[0016] In another aspect, the invention is directed to a chemicalmechanical polishing apparatus. The apparatus has a polishing pad with apolishing surface, a carrier to hold a substrate against the polishingsurface, a motor coupled to at least one of the polishing pad andcarrier head for generating relative motion therebetween, and aconductive layer thickness monitoring system. The conductive layerthickness monitoring system including at least one inductor, a currentsource that generates a drive signal, the current source electricallycoupled to the at least one inductor to induce an alternating current inthe at least one inductor and generate an alternating magnetic field,sense circuitry including a capacitor electrically coupled to the atleast one inductor to sense the alternating magnetic field and generatea sense signal, and phase comparison circuitry coupled to the currentsource and the sense circuitry to measure a phase difference between thesense signal and the drive signal.

[0017] Implementations of the invention may include one or more of thefollowing features. At least one first gate, e.g., an XOR gate, mayconvert sinusoidal signals from the inductor and the oscillator intofirst and second square-wave signals. A comparator, e.g., an XOR gate,may compare the first square-wave signal to the second square-wavesignal to generate a third square-wave signal. A filter may convert thethird square-wave signal into differential signal having an amplitudeproportional to the phase difference between the first and second squarewave signals. The phase comparison circuitry may generate a signal witha duty cycle proportional to the phase difference.

[0018] In another aspect, the invention may be directed to a method ofmonitoring a thickness of a conductive layer on a substrate during achemical mechanical polishing operation. In the method, a coil isenergized with a first signal to generate an alternating magnetic field.The alternating magnetic field induces eddy currents in a conductivelayer of the substrate. The alternating magnetic field is measured and asecond signal is generated indicative of the magnetic field. The firstand second signals are compared to determine a phase differencetherebetween.

[0019] Implementations of the invention can include zero or more of thefollowing possible advantages. The endpoint detector can sense thepolishing endpoint of a metal layer in-situ. The magnetic fieldapparatus for the endpoint detector can be embedded in the platen belowa polishing pad. The magnetic field apparatus can be protected frompolishing environment, e.g., corrosive slurry. The endpoint detectorneed not use complex electronics. Polishing can be stopped withreasonable accuracy. Overpolishing and underpolishing substrate can bereduced, thereby improving yield and throughput.

[0020] Other features and advantages of the invention will becomeapparent from the following description, including the drawings andclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic exploded perspective view of a chemicalmechanical polishing apparatus.

[0022]FIG. 2 is a schematic side view, partially cross-sectional, of achemical mechanical polishing apparatus including an eddy currentmonitoring system.

[0023]FIG. 3 is a schematic circuit diagram of the eddy currentmonitoring system.

[0024] FIGS. 4A-4C are schematic cross-sectional views of a polishingpad.

[0025]FIG. 5 is a schematic cross-sectional view illustrating a magneticfield generated by the monitoring system.

[0026]FIG. 6 is a schematic perspective view of a core from an eddycurrent sensor.

[0027] FIGS. 7A-7D schematically illustrating a method of detecting apolishing endpoint using an eddy current sensor.

[0028]FIG. 8 is a graph illustrating a trace from the eddy currentmonitoring system.

[0029]FIG. 9 is a schematic diagrams an eddy current monitoring systemthat senses a phase shift.

[0030]FIGS. 10A and 10B are schematic circuit diagrams of twoimplementations of an eddy current monitoring system of FIG. 9.

[0031]FIG. 11 is a graph illustrating a trace from the eddy currentmonitoring system that measures phase shift.

DETAILED DESCRIPTION

[0032] Referring to FIGS. 1 and 2A, one or more substrates 10 can bepolished by a CMP apparatus 20. A description of a similar polishingapparatus 20 can be found in U.S. Pat. No. 5,738,574, the entiredisclosure of which is incorporated herein by reference. Polishingapparatus 20 includes a series of polishing stations 22 and a transferstation 23. Transfer station 23 transfers the substrates between thecarrier heads and a loading apparatus.

[0033] Each polishing station includes a rotatable platen 24 on which isplaced a polishing pad 30. The first and second stations can include atwo-layer polishing pad with a hard durable outer surface or afixed-abrasive pad with embedded abrasive particles. The final polishingstation can include a relatively soft pad. Each polishing station canalso include a pad conditioner apparatus 28 to maintain the condition ofthe polishing pad so that it will effectively polish substrates.

[0034] A two-layer polishing pad 30 typically has a backing layer 32which abuts the surface of platen 24 and a covering layer 34 which isused to polish substrate 10. Covering layer 34 is typically harder thanbacking layer 32. However, some pads have only a covering layer and nobacking layer. Covering layer 34 can be composed of foamed or castpolyurethane, possibly with fillers, e.g., hollow microspheres, and/or agrooved surface. Backing layer 32 can be composed of compressed feltfibers leached with urethane. A two-layer polishing pad, with thecovering layer composed of IC-1000 and the backing layer composed ofSUBA-4, is available from Rodel, Inc., of Newark, Del. (IC-1000 andSUBA-4 are product names of Rodel, Inc.).

[0035] A rotatable multi-head carousel 60 supports four carrier heads70. The carousel is rotated by a central post 62 about a carousel axis64 by a carousel motor assembly (not shown) to orbit the carrier headsystems and the substrates attached thereto between polishing stations22 and transfer station 23. Three of the carrier head systems receiveand hold substrates, and polish them by pressing them against thepolishing pads. Meanwhile, one of the carrier head systems receives asubstrate from and delivers a substrate to transfer station 23.

[0036] Each carrier head 70 is connected by a carrier drive shaft 74 toa carrier head rotation motor 76 (shown by the removal of one quarter ofcover 68) so that each carrier head can independently rotate about itown axis. In addition, each carrier head 70 independently laterallyoscillates in a radial slot 72 formed in carousel support plate 66. Adescription of a suitable carrier head 70 can be found in U.S. patentapplication Ser. No. 09/470,820, filed Dec. 23, 1999, the entiredisclosure of which is incorporated by reference. In operation, theplaten is rotated about its central axis 25, and the carrier head isrotated about its central axis 71 and translated laterally across thesurface of the polishing pad.

[0037] A slurry 38 containing a reactive agent (e.g., deionized waterfor oxide polishing) and a chemically-reactive catalyzer (e.g.,potassium hydroxide for oxide polishing) can be supplied to the surfaceof polishing pad 30 by a slurry supply port or combined slurry/rinse arm39. If polishing pad 30 is a standard pad, slurry 38 can also includeabrasive particles (e.g., silicon dioxide for oxide polishing).

[0038] Referring to FIGS. 2A and 3, a recess 26 is formed in platen 24,and a thin section 36 can be formed in polishing pad 30 overlying recess26. Aperture 26 and thin pad section 36, if needed, are positioned suchthat they pass beneath substrate 10 during a portion of the platen'srotation, regardless of the translational position of the carrier head.Assuming that polishing pad 32 is a two-layer pad, thin pad section 36can be constructed as shown in FIG. 4A by removing a portion 33 ofbacking layer 32. Alternatively, as shown in FIG. 4B, thin pad section36′ can be formed by removing a portion 33′ of both backing layer 32′and a portion of cover layer 34′. Thus, this implementation has a recessin the bottom surface of cover layer 34 in the thin pad section 36. Ifthe polishing pad is a single-layer pad, thin pad section 36 can beformed by removing a portion of the pad material to create a recess inthe bottom surface of the pad. Alternatively, as shown in FIG. 4C, thinpad section 36″ can be formed by inserting a plug 37 of a differentmaterial into polishing pad 30. For example, the plug can be arelatively pure polymer or polyurethane, e.g., formed without fillers.In general, the material of pad section 36 should be non-magnetic andnon-conductive. If the polishing pad is itself sufficiently thin or hasa magnet permeability (and conductivity) that does not interfere withthe eddy current measurements, then the pad does not need anymodifications or recesses.

[0039] Returning to FIGS. 2A and 3, an in-situ eddy current monitoringsystem 40, which can function as an endpoint detector, includes a drivesystem 48 to induce eddy currents in a metal layer on the substrate anda sensing system 58 to detect eddy currents induced in the metal layerby the drive system. The monitoring system 40 includes a core 42positioned in recess 26 to rotate with the platen, a drive coil 44 woundaround one part of core 42, and a sense coil 46 wound around second partof core 42. For drive system 48, monitoring system 40 includes anoscillator 50 connected to drive coil 44. For sense system 58,monitoring system 40 includes a capacitor 52 connected in parallel withsense coil 46, an RF amplifier 54 connected to sense coil 46, and adiode 56. The oscillator 50, capacitor 52, RF amplifier 54, and diode 56can be located apart from platen 24, and can be coupled to thecomponents in the platen through a rotary electrical union 29.

[0040] Referring to FIG. 5, in operation the oscillator 50 drives drivecoil 44 to generate an oscillating magnetic field 48 that extendsthrough the body of core 42 and into the gap 46 between the two poles 42a and 42 b of the core. At least a portion of magnetic field 48 extendsthrough thin portion 36 of polishing pad 30 and into substrate 10. If ametal layer 12 is present on substrate 10, oscillating magnetic field 48generates eddy currents in the metal layer 12. The eddy currents causethe metal layer 12 to act as an impedance source in parallel with sensecoil 46 and capacitor 52. As the thickness of the metal layer changes,the impedance changes, resulting in a change in the Q-factor of sensingmechanism. By detecting the change in the Q-factor of the sensingmechanism, the eddy current sensor can sense the change in the strengthof the eddy currents, and thus the change in thickness of metal layer12.

[0041] Referring to FIG. 6, core 42 can be a U-shaped body formed of anon-conductive material with a relatively high magnetic permeability(e.g., μ of about 2500). Specifically, core 42 can be ferrite. In oneimplementation, the two poles 42 a and 42 b are about 0.6 inches apart,the core is about 0.6 inches deep, and the cross-section of the core isa square about 0.2 inches on a side.

[0042] In general, the in-situ eddy current monitoring system 40 isconstructed with a resonant frequency of about 50 kHz to 10 MHz, e.g., 2MHz. For example, the sense coil 46 can have an inductance of about 0.3to 30 μH and the capacitor 52 can have a capacitance of about 0.2 to 20nF. The driving coil can be designed to match the driving signal fromthe oscillator. For example, if the oscillator has a low voltage and alow impedance, the drive coil can include fewer turns to provide a smallinductance. On the other hand, if the oscillator has a high voltage anda high impedance, the drive coil can include more turns to provide alarge inductance.

[0043] In one implementation, the sense coil 46 includes nine turnsaround each prong of the core, and the drive coil 44 includes two turnsaround the base of the core, and the oscillator drives the drive coil 44with an amplitude of about 0.1 V to 5.0 V. Also, in one implementation,the sense coil 46 has an inductance of about 2.8 μH, the capacitor 52has a capacitance of about 2.2 nF, and the resonant frequency is about 2MHz. In another implementation, the sense coil has an inductance ofabout 3 μH and the capacitor 52 has a capacitance of about 400 pF. Ofcourse, these values are merely exemplary, as they are highly sensitiveto the exact winding configuration, core composition and shape, andcapacitor size.

[0044] In general, the greater the expected initial thickness of theconductive film, the lower the desired resonant frequency. For example,for a relatively thin film, e.g., 2000 Angstroms, the capacitance andinductance can be selected to provide a relatively high resonantfrequency, e.g., about 2 MHz. On the other hand, for a relativelythicker film, e.g., 20000 Angstroms, the capacitance and inductance canbe selected to provide a relatively lower resonant frequency, e.g.,about 50 kHz. However, high resonant frequencies may still work wellwith thick copper layers. In addition, very high frequencies (above 2MHz) can be used to reduce background noise from metal parts in thecarrier head.

[0045] Returning to FIGS. 2A, 2B and 3, the CMP apparatus 20 can alsoinclude a position sensor 80, such as an optical interrupter, to sensewhen core 42 is beneath substrate 10. For example, the opticalinterrupter could be mounted at a fixed point opposite carrier head 70.A flag 82 is attached to the periphery of the platen. The point ofattachment and length of flag 82 is selected so that it interrupts theoptical signal of sensor 80 while core 42 sweeps beneath substrate 10.Alternately, the CMP apparatus can include an encoder to determine theangular position of platen.

[0046] In operation, CMP apparatus 20 uses monitoring system 40 todetermine when the bulk of the filler layer has been removed and theunderlying stop layer has been exposed. Monitoring system 40 can as beused to determine the amount of material removed from the surface of thesubstrate. A general purpose programmable digital computer 90 can beconnected to amplifier 56 to receive the intensity signal from the eddycurrent sensing system. Computer 90 can be programmed to sampleamplitude measurements from the monitoring system when the substrategenerally overlies the core, to store the amplitude measurements, and toapply the endpoint detection logic to the measured signals to detect thepolishing endpoint. Possible endpoint criteria for the detector logicinclude local minima or maxima, changes in slope, threshold values inamplitude or slope, or combinations thereof.

[0047] Referring to FIG. 2B, the core 42, drive coil 44 and sense coil46 of the eddy current sensor located below thin section 36 of polishingpad 32 sweep beneath the substrate with each rotation of the platen.Therefore, the computer 90 can also be programmed to divide theamplitude measurements from each sweep of the core beneath the substrateinto a plurality of sampling zones 96, to calculate the radial positionof each sampling zone, to sort the amplitude measurements into radialranges, to determine minimum, maximum and average amplitude measurementsfor each sampling zone, and to use multiple radial ranges to determinethe polishing endpoint, as discussed in U.S. patent application Ser. No.09/460,529, filed Dec. 13, 1999, the entirety of which is incorporatedherein by reference.

[0048] Since the eddy current sensor sweeps beneath the substrate witheach rotation of the platen, information on the metal layer thickness isbeing accumulated in-situ and on a continuous real-time basis. In fact,the amplitude measurements from the eddy current sensor can be displayedon an output device 92 during polishing to permit the operator of thedevice to visually monitor the progress of the polishing operation.

[0049] Moreover, after sorting the amplitude measurements into radialranges, information on the metal film thickness can be fed in real-timeinto a closed-loop controller to periodically or continuously modify thepolishing pressure profile applied by a carrier head, as discussed inU.S. patent application Ser. No. 60/143,219, filed Jul. 7, 1999, theentirety of which is incorporated herein by reference. For example, thecomputer could determine that the endpoint criteria have been satisfiedfor the outer radial ranges but not for the inner radial ranges. Thiswould indicate that the underlying layer has been exposed in an annularouter area but not in an inner area of the substrate. In this case, thecomputer could reduce the diameter of the area in which pressure isapplied so that pressure is applied only to the inner area of thesubstrate, thereby reducing dishing and erosion on the outer area of thesubstrate. Alternatively, the computer can halt polishing of thesubstrate on the first indication that the underlying layer has beenexposed anywhere on the substrate, i.e., at first clearing of the metallayer.

[0050] Initially, referring to FIGS. 2A, 3 and 7A, oscillator 50 istuned to the resonant frequency of the LC circuit, without any substratepresent. This resonant frequency results in the maximum amplitude of theoutput signal from RF amplifier 54.

[0051] As shown in FIGS. 7B and 8, for a polishing operation, asubstrate 10 is placed in contact with polishing pad 30. Substrate 10can include a silicon wafer 12 and a conductive layer 16, e.g., a metalsuch as copper, disposed over one or more patterned underlying layers14, which can be semiconductor, conductor or insulator layers. Thepatterned underlying layers can include metal features, e.g., vias, padsand interconnects. Since, prior to polishing, the bulk of conductivelayer 16 is initially relatively thick and continuous, it has a lowresistivity, and relatively strong eddy currents can be generated in theconductive layer. As previously mentioned, the eddy currents cause themetal layer to function as an impedance source in parallel with sensecoil 46 and capacitor 52. Consequently, the presence of conductive film16 reduces the Q-factor of the sensor circuit, thereby significantlyreducing the amplitude of the signal from RF amplifier 56.

[0052] Referring to FIGS. 7C and 8, as substrate 10 is polished, thebulk portion of conductive layer 16 is thinned. As the conductive layer16 thins, its sheet resistivity increases, and the eddy currents in themetal layer become dampened. Consequently, the coupling between metallayer 16 and sensor circuitry 58 is reduced (i.e., increasing theresistivity of the virtual impedance source). As the coupling declines,the Q-factor of the sensor circuit 58 increases toward its originalvalue.

[0053] Referring to FIGS. 7D and 8, eventually the bulk portion ofconductive layer 16 is removed, leaving conductive interconnects 16′ inthe trenches between the patterned insulative layer 14. At this points,the coupling between the conductive portions in the substrate, which aregenerally small and generally non-continuous, and sensor circuit 58reaches a minimum. Consequently, the Q-factor of the sensor circuitreaches a maximum value (although not as large as the Q-factor when thesubstrate is entirely absent). This causes the amplitude of the outputsignal from the sensor circuit to plateau. Thus, by sensing when theamplitude of the output signal is no longer increasing and has leveledoff (e.g., reached a local plateau), computer 90 can sense a polishingendpoint. Alternatively, by polishing one or more test substrates, theoperator of the polishing machine can determine the amplitude of theoutput signal as a function of the thickness of the metal layer. Thus,the endpoint detector can halt polishing when a particular thickness ofthe metal layer remains on the substrate. Specifically, computer 90 cantrigger the endpoint when the output signal from the amplifier exceeds avoltage threshold corresponding to the desired thickness.

[0054] The eddy current monitoring system can also be used to trigger achange in polishing parameters. For example, when the monitoring systemdetects a polishing criterion, the CMP apparatus can change the slurrycomposition (e.g., from a high-selectivity slurry to a low selectivityslurry). As another example, as discussed above, the CMP apparatus canchange the pressure profile applied by the carrier head.

[0055] In addition to sensing changes in amplitude, the eddy currentmonitoring system can calculate a phase shift in the sensed signal. Asthe metal layer is polished, the phase of the sensed signal changesrelative to the drive signal from the oscillator 50. This phasedifference can be correlated to the thickness of the polished layer. Oneimplementation of a phase measuring device, shown in FIG. 10A, combinesthe drive and sense signals to generate a phase shift signal with apulse width or duty cycle which is proportional to the phase difference.In this implementation, two XOR gates 100 and 102 are used to convertsinusoidal signals from the sense coil 46 and oscillator 50,respectively, into square-wave signals. The two square-wave signals arefed into the inputs of a third XOR gate 104. The output of the third XORgate 104 is a phase shift signal with a pulse width or duty cycleproportional to the phase difference between the two square wavesignals. The phase shift signal is filtered by an RC filter 106 togenerate a DC-like signal with a voltage proportional to the phasedifference. Alternatively, the signals can be fed into a programmabledigital logic, e.g., a Complex Programmable Logic Device (CPLD) or FieldProgrammable Gate Array (FGPA) that performs the phase shiftmeasurements.

[0056] The phase shift measurement can be used to detect the polishingendpoint in the same fashion as the amplitude measurements discussedabove. Alternatively, both amplitude and phase shift measurements couldbe used in the endpoint detection algorithm. An implementation for boththe amplitude and phase shift portions of the eddy current monitoringsystem is shown in FIG. 10A. An implementation of the amplitude sensingportion of the eddy current monitoring system is shown in FIG. 10B. Anexample of a trace generated by an eddy current monitoring system thatmeasures the phase difference between the drive and sense signals isshown in FIG. 11. Since the phase measurements are highly sensitive tothe stability of the driving frequency, phase locked loop electronicsmay be added.

[0057] A possible advantage of the phase difference measurement is thatthe dependence of the phase difference on the metal layer thickness maybe more linear than that of the amplitude. In addition, the absolutethickness of the metal layer may be determined over a wide range ofpossible thicknesses.

[0058] The eddy current monitoring system can be used in a variety ofpolishing systems. Either the polishing pad, or the carrier head, orboth can move to provide relative motion between the polishing surfaceand the substrate. The polishing pad can be a circular (or some othershape) pad secured to the platen, a tape extending between supply andtake-up rollers, or a continuous belt. The polishing pad can be affixedon a platen, incrementally advanced over a platen between polishingoperations, or driven continuously over the platen during polishing. Thepad can be secured to the platen during polishing, or there could be afluid bearing between the platen and polishing pad during polishing. Thepolishing pad can be a standard (e.g., polyurethane with or withoutfillers) rough pad, a soft pad, or a fixed-abrasive pad. Rather thantuning when the substrate is absent, the drive frequency of theoscillator can be tuned to a resonant frequency with a polished orunpolished substrate present (with or without the carrier head), or tosome other reference.

[0059] Various aspects of the invention, such as placement of the coilon a side of the polishing surface opposite the substrate or themeasurement of a phase difference, still apply if the eddy currentsensor uses a single coil. In a single coil system, both the oscillatorand the sense capacitor (and other sensor circuitry) are connected tothe same coil.

[0060] The present invention has been described in terms of a preferredembodiment. The invention, however, is not limited to the embodimentdepicted and described. Rather, the scope of the invention is defined bythe appended claims.

What is claimed is:
 1. A method of monitoring a thickness of aconductive layer on a substrate during a chemical mechanical polishingoperation, comprising: energizing a coil with a first signal to generatean alternating magnetic field, the alternating magnetic field inducingeddy currents in a conductive layer of the substrate; measuring thealternating magnetic field and generating a second signal indicative ofthe magnetic field; comparing the first and second signals to determinea phase difference therebetween; and monitoring the phase difference forpolishing endpoint criteria.
 2. The method of claim 1, wherein the coilis energized with an oscillator.
 3. The method of claim 1, whereinmeasuring the alternating magnetic field includes sensing thealternating magnetic field with a second coil.
 4. The method of claim 3,wherein measuring the alternating magnetic field includes connecting thesecond coil in parallel with a capacitor.
 5. The method of claim 3,wherein energizing the coil uses the coil wound around a first portionof the core and measuring the alternating magnetic field uses the secondcoil wound around a second portion of the core.
 6. The method of claim1, wherein monitoring the phase difference for polishing endpointcriteria includes receiving in a computer an output signal derived fromcomparing the first and second signals.
 7. The method of claim 1,wherein comparing the first and second signals includes converting asinusoidal signal that generates the alternating magnetic field into afirst and second square-wave signals using at least one first gate. 8.The method of claim 7, wherein the at least one first gate is an XORgate.
 9. The method of claim 7, wherein comparing the first and secondsignals includes comparing the first square-wave signal to the secondsquare-wave signal to generate a third square-wave signal.
 10. Themethod of claim 9, wherein the comparator is an XOR gate.
 11. The methodof claim 9, wherein comparing the first and second signals includesconverting the third square-wave signal into a differential signalhaving an amplitude proportional to the phase difference between thefirst and second square wave signals using a filter.
 12. The method ofclaim 1, wherein comparing the first and second signals generates asignal with a duty cycle proportional to the phase difference.
 13. Themethod of claim 1, wherein the coil is located on a side of a polishingsurface opposite the substrate.